1. Field of the Invention
This invention relates to improvements in power supply topology. More particularly, the power supply circuit of this invention generates a bipolar output signal that is a combination of A.C. and D.C. voltages.
2. Description of Related Art
In printing and electrophotographic applications such as xerography, a charge retentive surface is electrostatically charged to form a latent image. The latent image is then developed and transferred to a substrate. Although the preferred embodiments are described in reference of xerography, the invention is applicable to any image forming apparatus that uses a toner developer to form an image on an electrostatically charged latent image.
Referring to FIG. 1, belt 10 moves in the direction of arrow 12 to advance successive portions of the belt sequentially through the various processing stations disposed about the path of movement.
Belt 10 is entrained about stripping roller 14, tension roller 16 and drive roller 20. Drive roller 20 is coupled to a motor (not shown) by suitable means such as a belt drive. Both stripping roller 14 and tension roller 16 are rotatably mounted. These rollers are also idlers which rotate freely as belt 10 moves in the direction of arrow 12.
At charging station A, a charging device 22 charges the photoreceptor belt 10 to a relatively high, substantially uniform negative potential.
At exposure station B, an original document is positioned face down on a transparent platen 30 for illumination with flash lamps 32. Light rays reflected from the original document are reflected through a lens 34 and projected onto a charged portion of photoreceptor belt 10 to selectively dissipate the charge thereon. The electrostatic latent image is formed on the belt 10.
Thereafter, belt 10 advances the electrostatic latent image to development station C. A hybrid scavengeless developer unit 80 generates a cloud of toner particles adjacent the electrostatic latent image. The latent image attracts the toner particles, thereby forming toner powder images on photoreceptor belt 10. If color images are formed, then a separate developer 80 for each color is needed.
At transfer station D, a sheet of support material such as a copy sheet (paper) is moved into contact with the developed latent image on belt 10. A supply tray 48, which may hold different quantities, sizes and types of support materials, supplies copy sheets along conveyor 46 and rollers 44. First, the latent image on belt 10 is exposed to a pretransfer light from a lamp (not shown) to reduce the attraction between photoreceptor belt 10 and the toner powder image thereon. Next a corona generating device, a transfer corotron 40, charges the copy sheet to the proper potential so that it is tacked to photoreceptor belt 10 and the toner powder image is attracted from photoreceptor belt 10 to the copy sheet. After transfer, a detack corotron 42 charges the copy sheet toward the opposite polarity to detack the copy sheet from belt 10, whereupon the sheet is stripped from belt 10 at stripping roller 14.
A cleaner module 26 is located before the charging station A. The cleaner module 26 removes excess toner from the belt 10.
Fusing station E includes a fuser assembly, which permanently affixes the transferred toner powder images to the copy sheets. Preferably, the fuser assembly includes a heated fuser roller 72 adapted to be pressure engaged with a pressure roller 74 (also called a back-up roller.) The toner powder image on the copy sheet is pressed against the heated fuser roller 72. In this manner, the toner powder image is permanently affixed to the copy sheet. The copy sheet is ejected.
The developer 80 will be described in more detail with reference to FIG. 2. A thin layer of non-magnetic toner is formed on the surface of the donor roller 84, which is usually a metal cylinder of aluminum or stainless steel having a dielectric coating. The magnetic roller 82 contacts the toner to charge the toner particles. The toner particles are then attracted to the donor roller 84. The amount of toner to be supplied to the donor roller 84 can be electrically controlled by a DC voltage applied by power source 96 with respect to the magnetic roller 82. The magnetic roller 82 can be electrically grounded or also charged by power supply 94 depending on the operating conditions required.
At the entrance of housing 88, powder cloud wires 92 are charged via power supply 98. An AC plus DC voltage differential is formed between the powder cloud wires 92 and the donor roller 84. The toner "flies" off of the donor roller 84 to form a cloud of toner.
A typical photosensitive surface is charged, for example, to approximately negative 800 volts. Light discharges sections of the photosensitive surface to form the latent image. The toner in the toner cloud is charged to approximately negative 700 volts to form a bias between the charges of the latent image and the donor roller. Toner flies across the gap from the entrance of the developer 80 to the discharged sections of the latent image on the belt 10. The gap is preferably under 300 .mu.m.
At least two separate power supplies of opposite polarity are used in these types of developers to provide the positive and negative voltages. An adding network is used to supply positive voltage during the load and develop cycles and negative voltage during the remaining portion of the print job cycle. Thus, the output of one power supply of each bipolar pair is wasted during the portion of the job cycle when it is not used. Further, approximately 75% of the power is consumed by the adding network. Therefore, the control of each developer wastes a large amount of power and demands that each such pair of internal power supplies be larger. Furthermore, each developer has duplicative parts, which add to the cost and maintenance of the developers. Beyond these paired bipolar D.C. power supplies, additional A.C. power supplies must be provided to supply the A.C. portions of the required developer biases.